diff options
| author | Yonghong Song <yonghong.song@linux.dev> | 2024-07-23 08:34:39 -0700 |
|---|---|---|
| committer | Andrii Nakryiko <andrii@kernel.org> | 2024-07-29 15:05:05 -0700 |
| commit | 92de36080c93296ef9005690705cba260b9bd68a (patch) | |
| tree | 2463bda13615838c7e437d86b35379555a8d9aee /kernel/bpf/verifier.c | |
| parent | f86601c3661946721e8f260bdd812b759854ac22 (diff) | |
bpf: Fail verification for sign-extension of packet data/data_end/data_meta
syzbot reported a kernel crash due to
commit 1f1e864b6555 ("bpf: Handle sign-extenstin ctx member accesses").
The reason is due to sign-extension of 32-bit load for
packet data/data_end/data_meta uapi field.
The original code looks like:
r2 = *(s32 *)(r1 + 76) /* load __sk_buff->data */
r3 = *(u32 *)(r1 + 80) /* load __sk_buff->data_end */
r0 = r2
r0 += 8
if r3 > r0 goto +1
...
Note that __sk_buff->data load has 32-bit sign extension.
After verification and convert_ctx_accesses(), the final asm code looks like:
r2 = *(u64 *)(r1 +208)
r2 = (s32)r2
r3 = *(u64 *)(r1 +80)
r0 = r2
r0 += 8
if r3 > r0 goto pc+1
...
Note that 'r2 = (s32)r2' may make the kernel __sk_buff->data address invalid
which may cause runtime failure.
Currently, in C code, typically we have
void *data = (void *)(long)skb->data;
void *data_end = (void *)(long)skb->data_end;
...
and it will generate
r2 = *(u64 *)(r1 +208)
r3 = *(u64 *)(r1 +80)
r0 = r2
r0 += 8
if r3 > r0 goto pc+1
If we allow sign-extension,
void *data = (void *)(long)(int)skb->data;
void *data_end = (void *)(long)skb->data_end;
...
the generated code looks like
r2 = *(u64 *)(r1 +208)
r2 <<= 32
r2 s>>= 32
r3 = *(u64 *)(r1 +80)
r0 = r2
r0 += 8
if r3 > r0 goto pc+1
and this will cause verification failure since "r2 <<= 32" is not allowed
as "r2" is a packet pointer.
To fix this issue for case
r2 = *(s32 *)(r1 + 76) /* load __sk_buff->data */
this patch added additional checking in is_valid_access() callback
function for packet data/data_end/data_meta access. If those accesses
are with sign-extenstion, the verification will fail.
[1] https://lore.kernel.org/bpf/000000000000c90eee061d236d37@google.com/
Reported-by: syzbot+ad9ec60c8eaf69e6f99c@syzkaller.appspotmail.com
Fixes: 1f1e864b6555 ("bpf: Handle sign-extenstin ctx member accesses")
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240723153439.2429035-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Diffstat (limited to 'kernel/bpf/verifier.c')
| -rw-r--r-- | kernel/bpf/verifier.c | 5 |
1 files changed, 3 insertions, 2 deletions
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index c419005a29dc..2ab59db5837f 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -5626,12 +5626,13 @@ static int check_packet_access(struct bpf_verifier_env *env, u32 regno, int off, /* check access to 'struct bpf_context' fields. Supports fixed offsets only */ static int check_ctx_access(struct bpf_verifier_env *env, int insn_idx, int off, int size, enum bpf_access_type t, enum bpf_reg_type *reg_type, - struct btf **btf, u32 *btf_id, bool *is_retval) + struct btf **btf, u32 *btf_id, bool *is_retval, bool is_ldsx) { struct bpf_insn_access_aux info = { .reg_type = *reg_type, .log = &env->log, .is_retval = false, + .is_ldsx = is_ldsx, }; if (env->ops->is_valid_access && @@ -6945,7 +6946,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn return err; err = check_ctx_access(env, insn_idx, off, size, t, ®_type, &btf, - &btf_id, &is_retval); + &btf_id, &is_retval, is_ldsx); if (err) verbose_linfo(env, insn_idx, "; "); if (!err && t == BPF_READ && value_regno >= 0) { |